The use of polymeric materials to manufacture molded articles, such as closure devices and containers, is generally known. Different methods may be employed to manufacture closure devices, such as bottle caps, or containers, such as bottles. For example, such closure devices may be manufactured via compression molding or injection molding processes; or in the alternative, containers may be manufactured via blow molding, injection blow molding, or injection stretch blow molding.
In compression molding process, a two-piece mold provides a cavity having the shape of a desired molded article. The mold is heated. An appropriate amount of molten molding compound from an extruder is loaded into the lower half of the mold. The two parts of the mold are brought together under pressure. The molding compound, softened by heat, is thereby welded into a continuous mass having the shape of the cavity. If the molding compound is a thermosetting material, the continuous mass may be hardened via further heating, under pressure, in the mold. If the molding compound is a thermoplastic material, the continuous mass may be hardened via chilling, under pressure, in the mold.
In injection molding process, molding compound is fed into an extruder via a hopper. The extruder conveys, heats, melts, and pressurizes the molding compound to a form a molten stream. The molten stream is forced out of the extruder through a nozzle into a relatively cool mold held closed under pressure thereby filing the mold. The melt cools and hardens until fully set-up. The mold then opens and the molded part is removed.
In blow molding process, for example injection blow molding, the molding compound is melted, and then, it is formed into a tube or parison. The ends of the tube or parison is sealed, except for an area in which the blowing air can enter. The sealed tube or parison is inflated inside of a mold thereby taking the shape of the mold. The molded article is cooled, and then ejected from the mold. If necessary, the molded article is then trimmed.
In general, a closure device, such as a soda bottle cap, should be strong enough to withstand the pressure of a carbonated drink, and yet soft enough to provide an excellent seal on the bottle without the need for an inner liner. Additionally, a closure device, such as a soda bottle cap, should generally possess good environmental stress crack resistance, good impact strength, good removal torque, and good strip torque. Different techniques have been employed to provide for such closure devices having acceptable properties.
For example, the use of a polypropylene polymer as a bottle cap closure for the needed strength with an inner liner, which may be comprised of soft ethylene/vinyl acetate (EVA), polyvinyl chloride (PVC), butyl rubber, etc., is also generally well known. However, this two-part construction is costly because of the need for an inner liner. Furthermore, it would be easier and more convenient to use a one-piece closure, without a liner.
In attempts to eliminate the need for a two-part construction, the use of different blends of polymers has been suggested. However, there is still a need for polymer formulations that can be molded into closure devices having acceptable properties, such as no need for liners to facilitate a seal, acceptable taste and odor, satisfactory stress crack resistance, and impact strength to prevent cap failure.